The present invention relates to an internal combustion engine directly injecting fuel into a combustion chamber thereof.
In the prior art, a direct injection type internal combustion engine having the piston top faces formed with a recess to define the combustion chamber was frequently used for large-sized engine applications because it had advantages over internal combustion engines having a swirl chamber or a precombustion chamber, in that it had less friction loss and lowered fuel consumption. These advantages stem from the fact that there is no communication hole between the swirl or precombustion chamber and the combustion chamber, and that the compression ratio is reduced to a relatively small value.
However, a small-sized engine having a small cylinder bore diameter encounters more problems than the large-sized engine.
In the direct injection type internal combustion engine of the prior art, more specifically, a fuel injector is arranged substantially at the center of the recess, which is formed in the piston top face, so that a plurality of fuel jects may be radially injected from a plurality of injection ports. The intake swirls established by an intake valve or in an intake passage in the suction stroke of the engine still exist even at the end of the compression stroke, so that an air-fuel mixture is prepared in the recess while the atomized fuel droplets are entrained in the swirling direction of the vortexes. The recess used customarily has a diameter ranging from 40% to 70% to that of the piston or the cylinder. In a small-sized engine having pistons of a diameter smaller than 100 mm, therefore, the recess diameter is accordingly small and is reduced all the more since it is intended to increase the compression ratio. As a result, the fuel jets, which are radially injected from the plural injection ports of the fuel injector, impinge upon the inner wall surface of the recess so that they wet the wall surface in the form of a fuel film, or remain in the form of coarse droplets, which fail to be effectively burned. As a result, the volume of mixture effective for combustion is reduced, to raise problems such as a reduction in the output power, an increase in the fuel consumption rate and/or the emission of smoke.
In order to prevent the fuel droplets from impinging upon the wall surface of the recess, there have been generally adopted a method (a) by which the swirling flow to be established in the combustion chamber is intesified; a method (b) by which the fuel injector has its injection portions reduced in size but increased in number; and a method (c) by which the compression ratio is increased to raise the pressure (or density) in the recess of the time of fuel spray to thereby reduce the penetration of the fuel injector.
In method (a), an engine having a cylinder bore diameter of 100 to 120 mm has its swirl ratio (which is a measure of the intensity of the swirl to be established in the combustion chamber and is the ratio of the swirling angular velocity to the engine r.p.m.) limited to about 4, whereas an engine having a cylinder bore diameter smaller than 90 mm has its swirl ratio limited to 3.5 to 3.6. If the swirl ratio exceeds the above-specified values, the intake passage has its flow resistance increased to invite problems such that the charging efficiency of the intake air by the engine is remarkably degraded and that the heat loss to the wall surface of the combustion chamber is augmented.
In method (b), the fuel is atomized to weaken its penetration if the effective areas of the injection ports of the fuel injector are reduced. With excessively small sizes, however, the injection ports are liable to become clogged, so that there arise practical problems for injection ports having a diameter smaller than 0.15 mm. If the number of the injection ports is excessively enlarged, on the other hand, the fuel droplets injected from adjoining injection ports collide with one another in the vicinity of the side wall of the recess so that an overrich region of fuel is locally established to raise problems causing engine smoking. An engine having a cylinder bore diameter smaller than 120 mm is usually equipped with four or five injection ports.
In method (c), since the compression ratio is determined by the ratio between the stroke volume and the clearance volume at top dead center, the volume of the recess has to be more than 70% of the clearance volume so that the direct injection type internal combustion engine may generate sufficient output power. In order to raise the compression ratio, therefore, it is desired that the clearance, having no contribution to combustion, formed between the cylinder head and the piston top face is minimized. However, the aforementioned clearance is limited to about 0.5 mm if the thermal expansion of the engine parts caused by combustion, production errors and so on are taken into consideration. As a result, it becomes more difficult to increase the compression ratio for a smaller engine. If the compression ratio could be raised, there then arises a problem such that the engine becomes difficult to assemble and adjust.